A Novel "Green" Fully-integrated Ultrasensitive RFID-enabled Gas SensorUtilizing Inkjet-printed Antennas and Carbon Nanotubes

L. Yang*, D. Staiculescu, R. Zhang, C.P. Wong, and M. M. TentzerisGeorgia Institute of Technology, Atlanta, GA, 30332, U.S.A.

Abstract

In this paper, the integration of a flexible RFID tag with an inkjet-printed Single WalledCarbon Nanotube (SWCNT) film in a chipless sensor node for toxic gas detection wasintroduced for the first time. The whole module is realized on a "green" low-cost papersubstrate. Carbon nanotube composites change their electric properties (e.g. resistance,dielectric properties) in the presence of very small quantities of toxic gases like ammoniaand nitrogen oxide. The RFID tag is designed for the 868 MHz UHF band. When NOxgas is present, the resistance of the SWCNT film will drop, resulting in the decrease ofthe backscattered power which will be detected by the RFID reader to realize the reliabletoxic gas detection in a non-invasive way.

Introduction

The increased environmental awareness requires more and more applications that usesimple, organic, recyclable solutions, fully integrated on flexible substrates for ease ofuse for almost all applications, including body area networks for medical systems,tracking for pharmaceutical and food industries, toxic pollutants detection, supply chain,space and many more. This demand is further enhanced by the need for lightweight,reliable and durable wireless RFID-enabled sensor nodes [1]. The choice of paper as thesubstrate material presents multiple advantages and has established paper as one of themost promising materials for UHF RFID applications: it is widely available, recyclable,and the high demand and the mass "reel-to-reel" production make it the cheapest materialever made. Furthermore, the expensive and very polluting batteries need to be replaced inmany applications, thus necessitating the emergence of low-cost power scavengingsolutions. Energy forms like solar, thermal, vibrational and pressure can be scavenged topower the sensor nodes for various applications. Previous work has demonstrated thesuccessful development of a fully inkjet-printed RFID module on paper [2]. The nextchallenge is to integrate the sensor and the scavenger on the paper substrate as well. Theapplication of interest for this work is wireless sensing of toxic gas. Single-WalledCarbon Nanotube (SWCNT) composites were found to have electrical properties highlysensitive to extremely small quantities of gases, such as ammonia (NH3), carbon dioxide(C02), nitrogen oxide (NOx), etc. at room temperatures with a very fast response time [3].

This paper presents, for the first time, a chipless CNT-based RFID-enable sensor nodefor gas sensing applications, fully printed directly on paper substrate. The printedSWCNT is from Carbon Solutions, which were dispersed in dimethylformamide (DMF)solution for the inkjet printer. The impedance of the SWCNT film forms the sensor part.The antenna was printed first, followed by the 20 layers of the dispersed SWCNT as aload. The expected backscattered power variation from the RFID is 5% due to theimpedance change in the presence of NOx. The gas detection is easily realized in anoninvasive way realized by just detecting this power level variation from the RFIDreader's side.

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Inkjet-printed SWCNT

As a direct-write technology, inkjet printing transfers the pattern directly to thesubstrate. Due to its capability of jetting one single ink droplet in the amount as low as1pI, it has widely drawn attention from the industrial world as a more accurate andeconomic fabrication method than the traditional lithography method.

A SWCNT ink solution was developed for the first time ever as the first step to enablethe SWCNT to be inkjet printed. The sample SWCNT powder was dispersed in DMF, apolar aprotic solvent. The concentration of the ink was 0.4mg/ml. The diluted solutionwas purified by sonicated for 12 hours to prevent aggregations of large particle residues.This is important to avoid the nozzle clogging by SWCNT flocculation during theprinting process. Dimatix Materials Printer DMP-2800 equipped with DMCLCP-11610printer head was used to eject the SWCNT ink droplet onto a flexible substrate. Fourdevices with 10, 15, 20 and 25 SWCNT layers were fabricated to investigate theelectrical properties. Fig. 1 shows the fabricated samples.

•••

Fig. 1. Photograph of the inkjet-printed SWCNT films with silver electrodes.

Antenna Design and Measurement

A passive RFID system operates in the following way: the RFID reader sends aninterrogating RF signal to the RFID tag consisting of an antenna and an IC chip as a load.The power reflection coefficient of the RFID antenna can be calculated as a measure toevaluate the reflected wave strength.

IS 21 =IZ/oad -ZANT *12

(1)Z/oad -ZANT

where ZLoad represents the impedance of the load and ZANT represents the impedance ofthe antenna terminals with ZANT* being its complex conjugate. The same mechanism canbe used to realize RFID-enabled sensors. The inkjet-printed SWCNT film functions as atunable resistor with a value determined by the existence of the target gas. The RFIDreader monitors the backscattered power level. When the power level changes, it meansthat there is variation in the load resistance, therefore the sensor detects the existence of thegas.

To verify the performance of the proposed topology, a bow-tie meander line dipoleantenna was designed and fabricated on a 100um thickness flexible paper substrate withdielectric constant 3.2. The nature of the bow-tie shape offers a more broadbandoperation for the dipole antenna. Since conductive ink has become the major cost of anRFID tag after introducing the paper-based substrate into the low-cost RFID tag designmethodology, it is naturally desirable to minimize the amount of ink used per antenna in

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the mass production. This is achieved by reducing the printing area in the weakest currentdensity region, which is the dark area in the bow-tie shape as shown in Fig. 2. TheSWCNT film was inkjet printed in the center.

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Fig. 2. The RFID tag module design on flexible substrate: (a) current density distribution of theproposed antenna configurations (b) photograph of the fabricated conformal tag.

The simulation and measurement results of the return loss of the proposed antenna areshown in Fig. 3 (a), showing a very good agreement. The tag bandwidth extends from810MHz to 890MHz, covering the whole European RFID band. The antennaperformance is plotted in Fig. 3 (b), which is almost omnidirectional at 868MHz withdirectivity around 2.01dBi and 94.2% radiation efficiency.

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(a) (b)Fig. 3. (a) Measured return loss of the SWCNT-enabled RFID tag antenna (b) Simulated far-fieldradiation pattern plot. An omnidirectional radiation pattern can be observed at <1>= 0° plane withdirectivity 2.01 dBi.

As a proof-of-concept benchmarking case, 20 layers of SWCNT were chosen to printon the gap of the fabricated RFID tag. In the air, the film performance is close to an800hm load with -I OdB return loss at 868MHz. When N02 is present, the return loss willbecome -13dB, assuming a 20% resistance decreasing. The return loss difference is

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equivalent to 5% of the energy arriving the RFID tag, which would not be reflected backto the reader, as shown in Fig. 3. By detecting this backscattered signal difference on thereader's side, the sensing function can be fulfilled.

Piezoelectric Scavenger for Active RFID-enabled SensorsThe tag presented in this paper is a passive one. However, state-of-the-art long-range

RFID tags are realized by active RFID topologies, which require a power supply. Untilnow, chemical-cell batteries have been sufficient, but replacing them is a costly nuisanceand an environmental disposal hazard, thus making this solution less practical as demandincreases. As the next step, extraction of electrical energy from human body movementwill be realized by the energy scavenger to power up the active RFID tag module onflexible organic substrate. The preliminary study investigates the feasibility of apiezoelectric scavenger, using a push-button piezoelectric switch. The schematic of thescavenging circuit is presented in Fig. 4. In experiment, the energy harvested from onebutton pushing, which can be trigged by walking movement, can power an eight-bitencoder to send two words. This proves that the concept is feasible and the next step willbe the integration of the scavenger on the paper substrate.

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Conclusions

The inkjet printing method has been utilized for the first time to deposit SWCNT filmon a fully-printed "green" RFID module on paper to form a wireless gas sensor node.Furthermore, the feasibility of a piezoelectric scavenger was successfully investigated tobe integrated in the tag. The last step will be the integration of the scavenger on the papersubstrate as well. Thus, the major steps have been completed for the development of anenergy-independent, trully "green" sensor node to help build the next generation ofenvironmentally-friendly electronics.

References

[1] T. Mishima, N. Abe, K. Tanaka, and H. Taki, "Toward construction of a mobile system with long-rangeRFID sensors" IEEE conference on Cybernetics and Intelligent Systems, vol. 2, pp. 960-965, 2004

[2] L. Yang, A. Rida, R. Vyas, M. M. Tentzeris, "RFID tag and RF structures on a paper substrate usinginkjet-printing technology," IEEE Transaction on Microwave Theory and Techniques, vol. 55, pp 2894­2901, Dec. 2007.

[3] K. G. Ong, K. Zeng, C. A. Grimes, "A wireless, passive carbon nanotube-based gas sensor, " IEEE Sens.Journal, 2002, vol. 2, pp 82-88.

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